Conventionally, smart key systems have been known. In a smart key system, when a user holding a mobile device (a smart key) performs a predetermined operation, for example, contacts a touch sensor on a door handle, or presses an engine start button, an in-vehicle device wirelessly transmits a request signal into the vehicle compartment or a limited area out of the vehicle compartment. Then, the in-vehicle device receives a response signal wirelessly transmitted from the mobile device that has received the request signal, and compares information for authentication included in the response signal, with information for authentication registered in the in-vehicle device, to authenticate the mobile device. In this way, it is possible to implement unlocking and locking the doors, starting the engine, and the like, based on authentication of the mobile device.
Concerning such a smart key system, a method of stealing a vehicle, called “relay attack”, has been known. Since a request signal is transmitted from an in-vehicle device so as to reach just within the vehicle compartment or a limited area out of the vehicle compartment, a mobile device held by a user positioned out of the area cannot receive a request signal. However, even if the user holding the mobile device is out of the area, a malicious third person may use a repeater for relaying a request signal transmitted from the in-vehicle device to the mobile device, and this make it possible for the third person to unlock the doors, to start the engine, and the like. This is called a “relay attack”. Therefore, various technologies have been proposed to prevent relay attacks (see, for example, Patent Document 1).
Patent Document 1 describes a mobile device that transmits a response signal in response to receiving a request signal from an in-vehicle device, by an antenna using a gain lower than a gain normally used when transmitting an unlock signal in a remote keyless entry function. According to a configuration in Patent Document 1, output of the response signal is controlled when transmitted from the mobile device in response to a request signal from the in-vehicle device. This makes it difficult for a relay attacker to steal the vehicle because even if the mobile device out of the communication range of the in-vehicle device can receive the request signal from the in-vehicle device by relaying, the response signal transmitted from the mobile device may be prevented from reaching the in-vehicle device.
Note that it is assumed in the present disclosure that a relay attack is performed in an aspect where a repeater is used for relaying a request signal wirelessly transmitted from an in-vehicle device, not in an aspect where a repeater is used for relaying a response signal wirelessly transmitted from a mobile device.